Acta Climatologica et Chorologica XLVII-XLVIII. [antikvár]

ACTA CLIMATOLOGICA ET CHOROLOGICA Universitatis Szegediensis, Tomus 47-48, 2014, 7-16 synoptic study of the heaviest snowfalls in poznan since 1960/61 to 2009/2010 E BEDNORZ Department of Climatology, Adam Mickiewicz University, ul. Dzi^gielowa 27, 61-680 Poznan, Poland E-mail: [email protected] Summary: Composite maps of the sea level pressure and 500 hPa geopotential heights means and anomalies were constmcted for the days with the high snow accumulation in Poznari. Similar maps of the air temperature at the isobaric level 850 hPa and of precipitable water content were presented. Additionally, 48-hours back trajectories of air masses for chosen days with the most effective snowfalls were constructed, using the NOAA HYSPLIT model. Negative anomalies of sea level pressure and 500 hPa heights, which mean low pressure systems spreading over Europe, are the basic condition of abundant snowfalls in Poznari. Snowfalls may appear as a result of fronts in the colder parts of Mediterranean cyclones with the dynamic warm and humid air of distant southern origm climbing upwards on the cooler and more stabile polar air masses from the north or east. The alternative location of snow-bringing low pressure systems is the Baltic Sea region. Key words: snowfalls in Poland, air circulation, synoptic conditions, back trajectories 1. INTRODUCTION Winter in the European moderate zone is a very changeable season, with mild periods alternating with the cold ones, the latest being often accompanied with snowfalls or even heavy snowstorms. These severe weather events may have a strong economic impact Abundant snowfalls, often simultaneous with low temperatures, may cause traffic hazards, communication problems, and power shortages; consequently, they can paralyze community life. The European community experienced such events for example in the end of November 2005 after short but strong snowstorms which took place in Germany and Western Europe. Dramatic events took place in Bavaria in January 2006 when a deep snow accumulation after heavy snowfalls crushed building structures, and caused the collapse of the roof of the ice rink, and also in Katowice (Poland), where the exhibition hall collapsed during the racing pigeon show. Both tragedies brought many deaths and injuries. Serious social and economic effects justify research into the synoptic reasons for heavy snowstorms, as it may be helpftil in forecasting such weather events. Snow occurrence is determined by the air temperature, precipitation and, indirectly, by the inflow of particular air masses. The atmospheric circulation is a driving factor for weather conditions in winter and it is responsible for the above mentioned intra-seasonal variability of the winter weather in the moderate zone. The aim of this study is to find the daily circulation patterns and daily synoptic conditions responsible for abundant snowfalls in Poznaii, resulting in daily increase in snow cover depth by at least 10 cm. Closer analysis of synoptic conditions of heavy IJ It